Many of the damaging clinical feature of coronary artery disease can be asymptomatic or present without warning. Active myocardial ischemia is an important functional expression of coronary atherosclerosis and underlies most of these clinical manifestations. Tests in hospital aimed at the assessment of risk can measure the activity of myocardial ischemia but do not consider ischemia during daily life. Electrocardiographic monitoring in apparently stable ambulatory patients has shown frequent asymptomatic myocardial ischemia with many new characteristics not seen in tests performed in hospital (e.g., events are mostly asymptomatic, surprisingly prolonged, not related to stress and show a diurnal rhythm similar to that reported for myocardial infarction). This study aims to quantify the activity of myocardial ischemia in ambulatory patients with coronary artery disease out of the hospital in order to: a) study the natural history of ischemia in relation to the occurrence of coronary events; b) examine the effects of all active ischemia out of hospital on prognosis; and c) to establish if monitoring of active ischemia provides incremental information about risk of coronary events a[art from usual testing. A prospective study n consecutive patients will characterize them according to history, symptoms, risk factors, exercise testing for myocardial ischemia and cardiac catheterization data. Serial 48-hour frequency modulated ambulatory monitoring of the electrocardiogram will be performed on and off standard medication at baseline and at 6 month intervals. The frequency and duration of all active ischemia out of the hospital will be related in a multivariate analysis to symptoms, clinical and laboratory data, and to the occurrence of coronary events such as death, myocardial infarction, hospitalization for unstable angina and the need for revascularization. The long-term aims of this project are to: a) test the importance of all ischemia out of hospital in apparently stable patients with coronary disease as a measure of risk for future coronary events that is more discriminating than conventional testing in hospital, and b) to provide an objective means for assessing the effects of new therapies. Out of hospital ambulatory monitoring has shown that asymptomatic ischemia is common and overlooked in the management of apparently stable patients with coronary disease. If the activity of ischemia affects prognosis, it may not be sufficient to treat symptoms alone and new goals may be necessary that aim to control all active ischemia during daily life.